UPDATED on January 26, 2018
A wide variety of residential foundation types, including monolithic slabs, crawl space foundations, and basement foundations, can lose heat due to poorly detailed insulation at the concrete footings. That’s because many construction details, including some details on the GBA Web site, fail to address thermal bridging through foundation footings.
There are several possible ways to address these thermal bridges, including:
- ignoring the problem (based on the theory that the heat leaks are trivial);
- on some types of foundations — those with stemwalls insulated on the exterior — switching to interior wall insulation to allow for an uninterrupted thermal barrier;
- altering the construction details to include insulation under the footing.
In general, deep footings will lose less heat during the winter than shallow footings. Whether the amount of heat leaking through a concrete footing is enough to worry about depends on your climate and your performance goals; if you hope to achieve the Passivhaus standard, such a thermal bridge is clearly a no-no.
Is there any reason NOT to put foam under a footing?
Traditional wisdom taught builders to place footings on undisturbed soil below the frost line. If footings are properly designed for the soil at your building site — an exercise based on either soil testing and engineering calculations or rules of thumb and local knowledge — then such undisturbed soil should be able to support the weight of the building, with a healthy margin of safety.
Engineers explain that good soils should be able to support 3,000 lbs. per square foot (20.9 psi). As it turns out, common extruded polystyrene (XPS) insulation like Dow Styrofoam or Owens Corning Foamular has a compressive strength of 25 psi. That’s more than many soils that are routinely used to support a footing and a house. Moreover, it’s possible to order XPS or EPS with densities exceeding 25 psi. (Dow will be happy to sell you XPS with a compressive strength of 40, 60, or 100 psi; these products are likely to exceed the performance requirements needed for a residential project.)
In short, the weight of a concrete footing plus a concrete wall plus a two-story house isn’t going to compress the foam insulation under a footing — not by a long shot. So logic doesn’t support the traditional assumption that foam is squishier than undisturbed soil. It isn’t.
Building scientist John Straube points out that when rigid foam supports a load, it can suffer from “creep” or deflection. “Over 50 years, the foam can shrink by 10%,” Straube notes. However, as long as the creep is consistent, the building sitting on the foam shouldn’t suffer harm. “The real problem isn’t settling, it is differential settlement,” says Straube.
(For further engineering notes related to the design of buildings that bear on rigid foam, see Comment #42 by John Klingel, and Comment #45 by Josh Golek, below. Note that for some projects, engineers will recommend aiming for a maximum of 1% deformation rather than 10% deformation. For a useful chart providing guidance on deformation of EPS, see . Here is a link to a GBA Q&A thread with further discussion of this issue: EPS foam for load-bearing applications.)
Postscript: GBA readers provided helpful information on this issue in a January 2018 Q&A thread. Here is the link: Sub-Slab Insulation Density.
In that thread, Michael Maines noted, “Foam is rated at 10% deformation at maximum load, as you note, per ASTM C518. If you have 1 inch of sub-slab insulation that may be OK, but if you have 4 to 6 inches, or more, it starts to add up quickly. Perhaps more importantly, that number does not take into account long-term creep, which is hard to define, but one company, Foam-Control, recommends a 3:1 safety factor to guard against creep.
“I’ve had structural engineers look at unconventional foundation details many times and they are always uncomfortable with any structural loads on foam; no matter what I spec they seem to push it up a notch. Understandable, since their license is on the line.
“That said, 15 psi is very likely safe, and what I usually specify, and 10 psi might even work. But if the project is going to need an engineer’s stamp I brace myself for the upgrade to 25 psi.
“That’s all for slabs with no loads on them. When there is going to be a structural load, the same concepts apply, but 25 to 40 psi foam is recommended, depending on the situation — mainly, how big is the load, how well is it distributed and what is the risk of the concrete failing.”
In the same Q&A thread, John Ranson noted, “Engineers are requiring the foam to have at least the same compressive strength as mediocre, soft soil. ASTM D6817 says that EPS 29 (same density as Type IX EPS but made for load-bearing use) can withstand 10.9 psi with no more than 1% deformation. That’s slightly more than 1500 psf, the lowest soil bearing capacity that still has prescriptive footings in the IRC.”
It’s been done
Many builders have successfully installed foam under residential footings. For example, Thorsten Chlupp, an Alaskan builder and author, has installed as much as 12 in. of 25-psi foam under residential footings.
Rigid foam is often used under the footings of buildings with frost-protected shallow foundations. For example, the NAHB Research Center publication, recommends the use of XPS under the footings of unheated buildings.
Convincing local code officials
Just because rigid foam can support more weight per square inch than excellent soil, doesn’t mean that local code officials will understand the use of rigid foam under footings. Several architects and builders have had to engage in negotiations over the issue with local code officials; among those who have been successful are several in Washington state, including of Seattle, in Olympia, and Dan Whitmore of Blackbird Construction and engineer Carissa Farkas, both of Seattle.
According to , Dan Whitmore “had a pretty big hoop to jump through when he went to the City of Seattle for a building permit for his Passive House. He wanted to use structural foam underneath the load bearing portion of his foundation, but that use had not been permitted in Seattle before on a residential project. Armed with a lot of research from Insulfoam (the structural foam manufacturer), the backing of his structural engineer Carissa Farkas, and a lot of persistence, they made a few requested tweaks to the plans, everyone was satisfied and the permit was issued.”
Insulated raft foundation systems
To avoid the problem of thermal bridging through concrete footings, many European Passivhaus builders are now using “insulated raft” foundation systems. As typically installed, an insulated raft is a load-bearing flat slab on grade. The slab has a uniform thickness rather than a thickened edge. The concrete thickness and the rebar schedule are designed to support the loads imposed by the perimeter walls and any interior bearing walls.
The EPS forms for an insulated raft foundation resemble a big rectangular tray. There is a continuous horizontal layer of rigid foam under the entire slab, as well as vertical insulation at the slab perimeter; all of the foam locks together. After the concrete is placed, the foam forms stay put, just like the forms of an ICF wall.
An insulated raft foundation differs from a frost-protected shallow foundation:
- Insulated raft foundations have a uniform thickness rather than a thickened edge.
- Unlike many frost-protected shallow foundations, insulated raft foundations always include a continuous horizontal layer of insulation under the entire slab.
- Insulated raft foundations usually have no wing insulation, depending instead on a deep layer of crushed stone to address the problem of frost heaving.
North American manufacturers of EPS forms for raft slabs
Three North American companies are now distributing rigid foam forms for raft slab foundations:
- , 45 South Centre Street, Merchantville, NJ 08109; Phone: 856-662-4909.
- , 103 Warner Drive, Long Sault, Ontario, Canada K0C 1P0; Phone: 866-299-7567 or 613-936-0518.
- , P.O. Box 359, Lincolnville, ME 04849; Phone: 207-342-2085.
European manufacturers of EPS forms for raft slabs
Among the European manufacturers of EPS forms for insulated raft foundations are , , and .
Isoquick forms are made of EPS manufactured by BASF. The foam panels interlock with an egg-carton configuration that the manufacturer calls “pyramid-shaped lugs.”
Isoquick forms can be used to assemble an insulation system that is either 5.9 inches (R-23) or 11.8 inches (R-46) thick.
Tomorrow’s Energy forms are rated at R-38. The insulation and the slab have a total thickness of about 15.8 inches.
For energy nerds obsessed with insulation details and the problem of thermal bridging, insulated raft foundations are aesthetically satisfying. I predict that the growing interest in the Passivhaus standard in the U.S. and Canada will eventually lure European manufacturers of insulated raft forms to begin distributing their products on this side of the Atlantic. North American builders who want to build an insulated raft foundation can choose from two approaches: they can purchase components from in New Jersey or in Canada, or they can cobble together foundation forms using ordinary XPS or EPS panels held in place by conventional removable foundation forms.
To see a video of workers installing rigid foam at a foam-under-footings job, click here.
Last week’s blog: “All About Glazing Options.”